Narrow-band tunable optical source

ABSTRACT

A laser system capable of providing light of high intensity is disclosed. This system includes a laser gain medium and three reflectors. A first reflector and a second reflector spaced from the first reflector define a laser cavity that contains the laser gain medium. The second reflector has a reflectivity (R 2 ) larger than the reflectivity (R 1 ) of the first reflector such that light emitted from the laser gain medium resonates in the laser cavity. A third reflector having a reflectivity (R 3 ) larger than the reflectivity of the first reflector (R 1 ) is spaced from the second reflector to define a resonant cavity external to the laser cavity. Light passes from the laser cavity to resonate in the external resonant cavity. Part of the light passes from the external resonant cavity to the laser cavity to optically lock the laser gain medium. The distance between the second and the third reflectors is adjustable to tune the resonant frequency of the external cavity.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/619,877 filed on Mar.20, 1996 now abandoned.

FIELD OF THE INVENTION

This invention relates to optical systems for the generation of laserradiation. More particularly, this invention relates to a diode-pumped,optically-locked laser with a linear optical cavity.

BACKGROUND

In many laser applications (for example, chemical sensing),high-intensity laser light is needed. One way to provide high-intensitylight is using light trapped inside an optical cavity. An optical cavityor resonator consists of two or more mirrored surfaces arranged so thatincident light may be trapped, bouncing back and forth between themirrors. In this way, the light inside the cavity may be many orders ofmagnitude more intense than the incident light.

In many applications, the optical gain medium (such as a helium neondischarge tube) is within the optical cavity. For a typical diode laser,the cavity mirrors are deposited directly on the diode gain mediumitself. For some applications, however, such as frequency-tuning andlinewidth-narrowing, one or both of the diode's facets isantireflection-coated and the diode is operated inside an optical cavitydefined by mirrors external to the diode. Although a diode gain mediamay be operated inside such a cavity, the low damage threshold of thediode's emission facet severely limits the amount of power build-up inthe cavity.

To overcome this limitation while still generating a large opticalfield, the diode laser may be placed outside of a separate high-finesseoptical cavity in which the diode laser radiation is trapped. Thisseparate cavity is referred to hereinafter as a "build-up" cavity. Diodelasers, however, emit radiation with an optical bandwidth that is muchlarger than that of a high-finesse build-up cavity. To achievesubstantial amplification of diode laser radiation in a build-up cavitythe diode laser must be forced to emit coherent radiation with abandwidth that approaches or matches that of the cavity at a cavityresonant frequency. This process is hereinafter called "opticallocking."

One way to reduce the bandwidth of diode lasers is to use all-electronicfrequency-locking of diode lasers. This technique, however, requiresvery fast servos, a large degree of optical isolation of the diode laserfrom the cavity, and sophisticated electronic control.

As an alternative, substantial linewidth reduction can be achieved withoptical feedback (i.e., passive) schemes. For example, Dahmani et al.,in "Frequency stabilization of semi-conductor lasers by resonant opticalfeedback," Optics Letters, 12, pp. 876-878 (1987), reported passiveoptical locking of a diode laser to a build-up cavity. In thistechnique, light from a diode laser is directed into a build-up cavity.If the light has a frequency matched to a cavity resonant frequency, thelight is trapped. A portion of the trapped light is then directed backinto the diode laser to act as a passive feedback mechanism, which locksthe frequency of the low-finesse diode laser to that of the high-finessebuild-up cavity, as well as reduces the diode emission bandwidth.

A shortcoming of systems similar to that of Dahmani et al. is that suchsystems employ weak optical locking: only a very minute portion of thelight in the build-up cavity is fed back to the diode laser. Thedisadvantage of the weak optical locking technique is that it stillrequires careful electro-mechanical control of both the magnitude andphase of the light fed back to the diode laser. Additionally, suchsystems contain at least four reflectors.

Passive all-optical locking of antireflection-coated diode lasers toexternal resonant cavities has recently been exploited extensively.Examples include frequency doubling (W. Lenth and W. P. Risk in U.S.Pat. No. 5,038,352, "Laser system and method using a nonlinear crystalresonator," August 1991; W. J. Kozlovsky et al., "Blue light generationby resonator-enhanced frequency doubling of an extended-cavity diodelaser," August 1994, vol. 65(5), pp. 525-527, Appl. Phys. Lett.),frequency mixing (P. G. Wigley, Q. Zhang, E. Miesak, and G. J. Dixon,"High power 467 nm passively-locked signal-resonant sum frequencylaser," Post Deadline Paper CPD21-1, Conference on Lasers andElectro-optics, Baltimore, Md., Optical Society of America, 1995), andchemical sensing (David A. King, et al., in U.S. Pat. No. 5,432,610,"Diode-pumped power build-up cavity for chemical sensing," July, 1995).King et al., (supra and incorporated by reference in its entiretyherein) describe several embodiments in which a diode laser is opticallylocked to an external resonant cavity. King et al. teach that there is abroad restriction on the diode current and additional components may berequired to eliminate off-resonance reflections for a system containingthree reflective elements.

To illustrate the difficulty of passive all-optical locking of diodelaser, a brief description of the physics of an optical cavity is givenin the following. As depicted in FIG. 1, two reflective surfaces 2 and 4(with reflectivities (reflection coefficients) R₁ and R₂ respectively)define a cavity 6. This cavity 6 has a comb of resonant frequencieswhere the comb spacing is c/2L (c is the speed of light in the cavityand L is the optical distance between the two reflective surfaces 2 and4).

Light incident on a linear cavity generally undergoes one of twopossible phenomena as depicted in FIG. 1. In FIG. 1A, the frequency ofthe incident light 8 is far from a cavity resonant frequency. Thus, theincident light 8 is simply reflected as reflected light 10 by surface 2.FIG. 1B depicts the situation when the incident light 8 is at (or verynear) a cavity resonant frequency. In this case, the incident light istrapped as an intracavity beam 12 between surfaces 2 and 4. The trappedlight additionally leaks through surfaces 2 and 4, affecting thereflected beam 10 and the transmitted beam 14 from the cavityrespectively. The leakage is out of phase with the incident beam 8, thuscausing a destructive interference with the portion of beam 10 that issimply and nonresonantly reflected from surface 2.

When the incident beam 8 is at a cavity resonant frequency, theeffective reflectivity (reflection coeffieient) of the cavity 6 is lowerthan the simple nonresonant reflectivity (or reflection coefficient) ofsurface 2. This effect is shown in FIG. 1C, in which the reflectivity ofthe cavity (I_(ref) /I_(inc)) shown in FIG. 1A and FIG. 1B is plotted asa function of normalized frequency. The frequency is normalized to acomb spacing of the cavity such that a cavity resonance occurs for eachintegral value of normalized frequency. The cavity bandwidth is the fullwidth at half maximum of each resonance and becomes smaller as thereflectivities of surfaces 2 and 4 decrease. When R₁ equals R₂, themagnitude of the resonant and nonresonant reflections from surface 2 areequal and their phases differ by 180°. In this way, the cavityreflectivity drops to zero (in the absence of scattering) on a cavityresonance.

The goal of all-optical locking of a diode laser to a cavity is togenerate intracavity beam 12 with incident beam 8 from the diode laser.This imposes desirable optical properties (for example, bandwidth andfrequency) that originate from the cavity on the diode laser. Thereflected beam 10 from the cavity is used to frequency-lock the diodelaser to a cavity resonance. However, FIG. 1C shows that the reflectedbeam 10 is the weakest at a cavity resonance. Thus, it appears that, byoptical feedback, as the diode current is increased, the laser tends toreach threshold at a frequency other than a cavity resonant frequency.Therefore, it has long been believed by those skilled in the art thatthe structure shown in FIG. 1A is highly unsuitable forfrequency-locking of a diode laser.

Various approaches have been used to reduce the destructive interferencementioned above and to ensure that the most intense reflection back intothe diode laser originates uniquely from the optical cavity. A simpleapproach is to use additional cavity reflectors or reflections thatallows spatial isolation of the resonant feedback (Dahmani et al.,"Frequency stabilization of semiconductor lasers by resonant opticalfeedback," supra). Other solutions rely on using very small feedbackinto the diode laser from mirror-induced birefringence (C. E. Tanner, etal., "Atomic beam collimation using a laser diode with a self lockingpower-build-up cavity," May 1988, vol. 13 (5), pp. 357-359, OpticsLetters) or very weakly excited counter-propagating modes (A. Hemmerichet al., "Second-harmonic generation and optical stabilization of a diodelaser in an external ring resonator," April 1990, Vol. 15 (7), pp.372-374, Optics Letters). However, such additional reflectors tend toincrease the complexity and expense of constructing the laser system.

In addition to having high intensity, it is desirable that narrow-bandlaser light be tunable to different frequencies. Tunable narrow-bandoptical sources have many applications in fields such as spectroscopyand communications (e.g., atomic beam collimation, see, e.g., C. E.Tanner, B. P. Masterson, and C. E. Wieman, "Atomic beam collimationusing a laser diode with a self locking power-buildup cavity," OpticsLetters, vol. 13, 1988, p 357). Traditional sources of tunable radiationsuch as dye lasers are structurally complex and relatively large. Forthis reason they are not widely used. Diode lasers are simple, small andsomewhat tunable, however, they do not emit narrow-band radiation.

While noise sources increase the effective bandwidth of laser sources,the fundamental lower limit is determined from quantum fluctuations. Thebandwidth of a laser is expressed by the Schalow-Townes limit, which fordiode lasers is modified to (A. Yariv, Optical Electronics, Holt,Keinhart and Winston, Philadelphia, 4th Ed., 1991, p383): ##EQU1## whereν₀ is the lasing frequency, μ is the upper level laser populationdivided by the population inversion, h is Planck's constant, P is theoutput power of the atoms, and α is the ratio of real to imaginary partsof the refractive index of the diode material. The bandwidth of thecavity without a gain element is given by: ##EQU2## where δ is the sumof intracavity power losses (assumed to be small), n is the real part ofthe refractive index of the diode laser, c the speed of light, and L thecavity length (i.e. the optical distance between the reflectivesurfaces).

Typically diode lasers have an emission bandwidth in the 10 MHz to 100MHz range. Conversely, the quantum linewidth of gas lasers can be in the10⁻³ Hz range. The difference lies in the typical length of the cavity,which in the case of diode lasers is a few hundred microns. For gaslasers it is tens to hundreds of centimeters. Several mechanisms can beused for deceasing the bandwidth of a diode laser.

Increasing the output power is not preferable because an increase ofseveral orders of magnitude is required and the low damage threshold ofdiode lasers limits the power of individual diodes to moderately lowvalues. A better alternative is to increase the length of the diodecavity. This is achieved most often by antireflection coating theemission facet of the diode laser and placing an external reflector at arelatively large distance from the diode. This type of laser is usuallytermed an external cavity diode laser. Using this approach, researchershave reported bandwidths of 10 kHz (R. Wyatt and W. J. Devlin, "10 kHzlinewidth 1.5 μm InGaAsP external cavity laser with 55 nm tuning range,"Electronics Letters, vol. 19, 1983, p 110). Tunability of this device isachieved by using a reflection grating as the external reflector (e.g.,Hewlett-Packard Journal, February, 1993). However, an increase in cavitylength (typically in the range of one or a few tens of centimeters)results in an increase in longitudinal cavity mode density. As thegrating is tuned, longitudinal mode hopping (an unsuitable discontinuityin the tuning curve) is very often encountered.

A third way of narrowing the bandwidth is to couple the diode laser toan external resonant cavity (C. E. Tanner, B. P. Masterson, and C. E.Wieman, "Atomic beam collimation using a laser diode with a self lockingpower-buildup cavity," Optics Letters, vol. 13, 1988, p 357, supra).Frequency tuning is achieved by adjusting the distance between thecavity mirrors, thereby tuning the cavity resonant frequency, given byEquation 3 (see, Anthony E. Siegman, Lasers, University Science Books,Mill Valley, Calif., 1986, p. 432): ##EQU3## where m is the (very large)mode number (in the visible part of the spectrum m is a very largenumber). In this case the linewidth is determined by the bandwidth ofthe external cavity and can be very narrow because the mirrors can behighly reflective and Δν_(cav) is therefore very small. This approachrequires that the diode laser generates radiation at the frequency andwith the bandwidth of the resonant cavity. One technique is byfrequency-locking. The frequency-locking technique employed by Tanner etal. used weak optical feedback and therefore required two piezo stacks,one to adjust the cavity resonant frequency and the second to adjust thephase of light between the diode laser and the cavity. This technique iscomplicated, requiring careful servo control of the two piezo elements.What is needed is a tunable, frequency-locked laser with a relativelysimple structure and yet capable of generating high-intensity light.

SUMMARY

The present invention provides a laser system that has a first resonantcavity and a second resonant cavity having a common reflector betweenthem wherein the cavity length of the second resonant cavity isadjustable to tune the resonance frequency. These resonant cavities aredefined herein respectively as the "laser cavity" and the "externalresonant cavity" (or simply the "external cavity"). Typically, the laserbuild-up system contains three reflectors: a first reflector having areflectivity (R₁), a second reflector spaced from the first reflector todefine the laser cavity, and a third reflector spaced from the secondreflector to define the external cavity. The second reflector has areflectivity (R₂) larger than the reflectivity (R₁) of the firstreflector. The third reflector also has a reflectivity (R₃) larger thanthe reflectivity of the first reflector (R₁). A laser gain medium iscontained in the laser cavity to emit light to resonate in the lasercavity. Light passes from the laser cavity to resonate in the externalresonant cavity. Part of the light passes from the external resonantcavity back into the laser cavity to optically lock the laser gainmedium to a selected frequency.

Using such a system, a method for producing a high-intensity laser lightis provided. In this method, light emitted from the laser gain mediumresonates in the laser cavity and enters the external resonant cavity,resonating therein to reach high intensity. Part of the resonant lightin the external resonant cavity is transmitted through the secondreflector back to the laser cavity to lock optically the laser gainmedium to a resonant frequency of the external resonant cavity by strongoptical feedback. The cavity length of the external resonant cavity isadjustable to tune the resonant frequency.

In contrast to prior art external cavity diode lasers, in the presentinvention, the reflectivity of the second reflector (R₂) is not made tobe smaller than that of the first reflector (R₁). Due to the selectionof the relative values of R₁, R₂, and R₃, the frequency bandwidth of thelaser cavity is larger than that of the external cavity. In this laserbuild-up system, the narrow-bandwidth external cavity dominates thelaser gain medium by optical feedback. In this way, a narrow band laserlight source can be made. Unlike conventional passive-locking lasersystems, in the present invention, stable operation is achievable with asizeable amount of resonance feedback to lock the laser gain medium tothe resonant frequency of the external cavity. This is referred to as"strong feedback" locking. With a strong feedback locking, a relativelysimple mechanism can be used to tune the resonance of laser system toachieve a narrow bandwidth.

However, unlike conventional laser build-up systems using relativelylarge feedback locking (such as Lenth and Risk or Kozlovsky), whichrequire additional optical elements (such as mirrors) to facilitatestability, the present laser build-up system requires no such additionalelements for added stability. It is common knowledge that additionaloptical elements require alignment and complicate the manufacturingprocess, as well as increase the cost for components.

Furthermore, in the present invention, because the laser gain medium isnot located inside the external cavity, a very high-intensity (power)light can be present in the external cavity without causing damage tothe laser gain medium. The high reflectivity of the reflectors enableslight to be reflected in multiple passes in the extemal cavity, therebyallowing a narrow bandwidth without requiring a long cavity length. Withthis invention, a high-intensity laser light source can be made withvery few components (including optical elements, such as reflectors, andelectromechanical elements to fine-tune the position of the opticalelements). The intensity in the external resonant cavity can be one ormore orders of magnitude higher than that of the laser cavity and can be10 to 10⁵ as high as that emitted by the gain medium. Additionally, thenarrow bandwidth external cavity has a temporal averaging effect on thediode emission, minimizing fast fluctuations (the external cavity can bethought of as an optical capacitor). Therefore, the present invention isuniquely suitable to provide a compact high-intensity light source.

The high-intensity light made available in the laser build-up system ormethod of the present invention has a variety of applications. Examplesinclude but are not limited to the following: (1) diode laser modecleanup--where a well characterized output beam is required from one ormore solid state sources; (2) chemical sensing (e.g., as described byKing et al., supra, and U.S. Pat. No. 5,437,840 (King et al.)); particlecounting; nonlinear frequency generation (e.g., using a nonlinear mediuminside the external cavity); environmental sensing; and distancemeasurement.

BRIEF DESCRIPTION OF THE DRAWING

The following figures, which show the embodiments of the presentinvention, are included to better illustrate the present invention. Inthese figures, like numerals represent like features in the severalviews.

FIG. 1A is a schematic representation of light incident on an opticalcavity without resonance.

FIG. 1B is a schematic representation of light incident on an opticalcavity with resonance.

FIG. 1C is a graphical representation of cavity reflection related tonormalized frequency showing the effect of reflectivities of thereflectors in an optical cavity.

FIG. 2 is a schematic representation of an embodiment of a strongfeedback laser system of the present invention.

FIG. 3 is a schematic representation of another embodiment of a strongfeedback laser system of the present invention, having a mode-matchingdevice.

FIG. 4A is a schematic representation of yet another embodiment of astrong feedback laser system of the present invention, wherein thereflecting surfaces of the laser cavity is on the gain medium.

FIG. 4B is a schematic representation of an embodiment of a strongfeedback laser system of the present invention that has more than onegain medium.

FIG. 5 is a schematic representation of yet another embodiment of astrong feedback laser system of the present invention with afrequency-limiting device.

FIG. 6 is a schematic representation of an embodiment of afrequency-limiting device applicable in the present invention.

FIG. 7 is a graphical representation of the threshold current of thegain medium relating to laser-cavity length and external-cavity lengthwith a frequency-limiting device.

FIG. 8 is a schematic representation of yet another embodiment of astrong feedback laser system of the present invention, showing anonlinear crystal in the external cavity.

FIG. 9 is a schematic representation of yet another embodiment of astrong feedback laser system of the present invention, showing nonlinearcrystal with reflective surfaces formed thereon.

FIG. 10 is a schematic representation of yet another embodiment of astrong feedback laser system of the present invention, wherein thereflective surfaces are deposited on a solid support.

FIG. 11 is a schematic representation of an embodiment of the lasersystem of the present invention, showing a sample being analyzed and adetector for detecting light interaction by the sample.

FIG. 12 is a schematic representation of an embodiment of the lasersystem of the present invention, showing a piezoelectric stack foradjusting the distance between the second and the third reflectors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the present invention, a second reflective surface (or reflector)having a relatively large reflectivity is disposed between a firstreflective surface (or reflector) and a third reflective surface (orreflector) to define a laser cavity (which contains a laser gain medium)and an external cavity. Light resonates in, and passes back from, theexternal cavity to provide feedback to lock the laser gain medium at theresonant frequency of the external cavity. The distance between thesecond and the third reflectors is adjustable to tune the resonantfrequency of the external cavity, thereby providing a narrow-bandtunable light source.

For the sake of clarity, embodiments of a laser system with strongfeedback locking of the gain medium is described first. To theseembodiments, a mechanism can be provided to adjust the distance betweenthe second and the third reflectors (e.g., as shown in FIG. 12).

A first embodiment of the strong feedback laser build-up system is shownin FIG. 2. Three reflective surfaces (or reflectors) 101, 102, and 104are arranged in the laser system 100 so that light may travel betweenthem on a straight light path (represented by axis or line 106). Thereflectivities of the three surfaces 101, 102, and 104 are R₁, R₂, andR₃, respectively. A laser cavity, 108 (in this case a two-mirror opticalcavity) is defined between reflective surfaces (or reflectors) 101 and102. Reflective surfaces 102 and 104 define another two-mirror cavity,the external cavity 106. An optical gain medium 114 is disposed in thelaser cavity 108 in such a way that it amplifies light traveling alongaxis 106 without introducing any substantial additional reflection. Thiscan be done by antireflection-coating the facets of the gain medium 114to eliminate reflection. One alternative way to avoid reflection fromthe gain medium is to chamfer its facet(s). When the values of R₁, R₂,and R₃ are chosen according to the invention, and the gain mediumexhibits an optical nonlinearity, then above laser threshold the lightin both cavities 108 and 110 has the same optical properties (e.g.frequency and bandwidth). The optical properties of the light in thelaser cavity 108 are determined by the light in the external cavity 110.

A significant portion of the light incident on the external cavity 110passes back into the laser cavity 108 through the reflective surface 102to optically lock the gain medium. Depending on the application and theamount of light leaving the external cavity (e.g., through thereflective surface 104), the amount of light returning to optically lockthe gain medium can vary. Generally, this amount is about 3% to about90%; and preferably, because of optical limitations of commonlyavailable optical elements, about 10% to about 50%. Thus, this resultsin strong optical feedback for optical locking of the gain medium to theresonant frequency of the external cavity. A suitable gain medium hasadequate nonlinearity such that it can be optically locked to theexternal cavity with strong optical feedback. Although, because of itslarge nonlinearity, diode laser is the preferred gain medium, othernonlinear gain media, such as titanium-doped sapphire, organic material,and the like, can be used.

The bandwidth of an optical cavity is determined by the reflectivitiesof the cavity mirrors. In this invention the reflectivities R₂ and R₃are chosen to be much higher than reflectivity R₁. With suchreflectivities, the bandwidth of the external cavity 110 is severalorders of magnitude smaller than the bandwidth of the laser cavity 108.The laser cavity length and the external cavity length are the opticaldistances between surfaces 101 and 102 and between surfaces 102 and 104,respectively.

In this invention, the value of R₁ is generally about 0.1 to about 0.99,R₂ is about 0.9 to about 0.999999, and R₃ is about 0.9 to about0.999999. For some applications, such as chemical analysis (e.g.,similar to the technique used in King et al, U.S. Pat. No. 5,432,610),preferably, to provide higher intensity light in the external cavity, R₁is about 0.1 to about 0.99, R₂ is about 0.995 to about 0.999999, morepreferably about 0.995 to 0.9999, whereas R₃ is about 0.995 to about0.999999, more preferably about 0.995 to 0.99999. For some otherapplications, such as intracavity nonlinear generation of light, thepreferable values are between 0.1 and 0.99 for R₁, between 0.9 and0.999999 for R₂, and between 0.9 and 0.999999 for R₃. In addition, tohave stronger optical feedback, it is preferable that R₁ is less thanR₂, which is preferably less than R₃ (i.e., R₁ <R₂ <R₃). However, theoptically-locked system will still function if R₂ is larger than orequal to R₃. In practice (using real components), R₂ and R₃ may be equalyet the reflectivity of the cavity is nonzero due to scattering loss oflight. The reflectivity values of the reflectors are selected tooptimize both the optical locking stability as well as the desiredamount of power that is accessed by emission through the third reflectorsurface 106.

In conventional external cavity diode lasers (ECL) (such as thosedescribed by Jens Buus, Single frequency semiconductor lasers, SPIEOptical Engineering Press, Bellingham, Wash. 1991, Section 8.2) surfaces101 and 102 are the facets of a diode laser. It is well known that, forstable operation, the reflectivity R₂ is made to be as small aspossible--orders of magnitude less than R₁ (P. Zorabedian, "Axial-modeinstability in tunable external-cavity semiconductor lasers," July 1994,vol. 30 (7), pp. 1542-1552, IEEE Journal of Quantum Electronics). It isalso well known that when R₃ is large and R₂ approaches R₁ the systemshown in FIG. 2 will enter the coherence collapse regime (J. Buus,supra) and operation becomes unstable. Instability is marked by adissimilarity of optical (phase) properties of the light in the twocavities 108 and 110 and usually results in linewidth broadening. Underconditions of higher feedback (e.g., more than 10%) the diode laseroperates stably only if the emission facet is antireflection-coated (R.W. Tkach, and A. R. Chraplyvy, "Regimes of feedback effects in 1.5 μmdistributed feedback lasers," November, 1986, vol. LT-14 (11), pp.1655-1661, Journal of Lightwave Technology). When such a emission facetis antireflection-coated, the laser system is, in effect, a two mirrorlaser system.

However, surprisingly, in the two cavity laser system of the presentinvention, stable operation is achieved when R₂ is much larger than R₁(i.e., its corresponding transmission is orders of magnitude larger thanthat of R₁). In fact, by choosing the reflectivities R₁, R₂, and R₃according to this invention, an entirely new operating regime isreached. Moreover, the performance of this device is much better thanthat of a conventional ECL because the linewidth can be much narrower ina more compact design and the beam shape is the more desirable lowestorder Hermite-Gaussian mode TEM₀₀. Stable operation is accomplished inthe present invention by judiciously selecting the reflectivities of thereflective surfaces in the laser build-up system.

In this invention, two resonant cavities (laser cavity and externalcavity) are separated by a common reflective surface, e.g., surface 102.The reflectivity R₁ is much smaller than R₂ and R₃. It is well knownthat the bandwidth of a simple two-mirror cavity depends on the mirrorreflectivities--the higher the reflectivities, the smaller thebandwidth. Thus, the bandwidth of the laser cavity 108 is much largerthan that of the external cavity 110. Under broad-band illumination, thecirculating electric field in the laser cavity 108 can be viewed as thesum of two components; one with a large bandwidth (originating in thelaser cavity) and the other with a small bandwidth (originating in theexternal cavity 110 and leaking through mirror 102). For the light inthe laser cavity 108 to have the same optical properties as the light inthe external cavity 110, the narrow bandwidth component must dominate asthe gain of the gain medium 114 is increased towards laser threshold.

FIG. 1C shows that for a cavity with R₁ =0.4 and R₂ =0.9 (curve C1) thereflectivity of a cavity at a cavity resonance may be 60% that of thefrom mirror (having a reflectivity of R₁). Curve C2 shows thereflectivity for a cavity with R₁ =R₂. For a system as shown in FIG. 2,where R₁ =0.85, R₂ =0.99936, and R₃ =0.99999, the reflectivity of theexternal cavity 110 at the cavity resonance can be calculated to be 94%that of the front mirror (R₂). However, for laser cavity length 5 cm andexternal cavity length 9 cm, the external cavity bandwidth is almost 280times smaller than that of the laser cavity. It is known that thethreshold inversion density for laser action is inversely proportionalto the cavity bandwidth (A. E. Siegman, Lasers, University ScienceBooks, Mill Valley, Calif., 1986, p. 511). The light with thenarrow-bandwidth from the external cavity will reach lasing threshold ata lower optical gain than the broad-bandwidth laser cavity component.Thus, the gain medium will be dominated by the feedback from theexternal cavity 110 rather than the simple reflection from surface 102.Although the above theory is believed to be correct, the operation andconstruction of the laser systems of the present invention ispracticable and do not depend on any particular theory.

FIG. 3 shows another preferred embodiment. Here, the gain medium isincorporated into the structure of a semiconductor diode laser 214. Theback facet of the laser is coated to be reflective and forms surface201. The emission facet 203 of the diode laser is antireflection (AR)coated, with reflectivity preferably in the range of less than 10⁻³.Reflective surfaces 202 and 204 are coated onto mirrors (substrates) 207and 209, respectively. These surfaces have appropriate curvatures tosupport a stable spatial mode in external cavity 210 (between surfaces202 and 204). Mode-matching optics 216 (e.g., lenses and/or prisms) wellknown to those skilled in the art can be used to spatially match thediode emission mode pattern into the external cavity 210. The surface219 of mirror (substrate) 207 facing the laser cavity 208 is preferablyantireflection-coated with a reflectivity in the range about 0.04 to0.001, preferably less than 0.02. Alternatively, the surface 219 can bea chamfer at an angle with the light path 206 to reduce its lightreflection into the gain medium.

As an example, such a system can be constructed using a Philips CQL801Ddiode laser as the gain medium 214 having emission facet 203 coated tohave a reflectivity within the range of 10⁻⁵ to 10⁻⁴. The mirror 207 and209 (having surfaces with reflectivities R₂ =R₃ =0.99999) can beobtained from Research Electro-optics, Boulder, Colo. The radius ofcurvature of each of the surfaces 202 and 204 forming the externalcavity is 5 cm. The mode-matching optics consists of an AR-coatedgradient index lens (GRIN lens) with a 0.23 pitch and a 5 cm focallength mode-matching lens. The external cavity length is 2 cm and thelaser cavity length is 4 cm. With a diode current of about 70 mA(obtained from a 9V transistor battery) stable continuous wave (CW)operation with about 145 W total power generated in the external cavityin a TEM₀₀ mode was obtained in such a system.

The optimum value of R₂ depends on a trade-off between the desired powerin the external cavity and the feedback (or the system stability) to thegain medium. For example, if the optical loss of the mode-matchingoptics (or any other optical component in the laser cavity) is large andsurface 203 is not perfectly antireflection-coated, to achieve stablesystem performance in the high feedback regime, more light must leakfrom the external cavity into the laser cavity. This can be accomplishedby reducing the value of R₂ while keeping the value of R₃ constant (seeFIG. 1C). However, at the same time the power in the external cavitywill decrease. In practice, the optimum value of R₂ depends on theoptical loss and the degree of mode match.

In another embodiment (FIG. 4A), both reflective surfaces of the lasercavity can be put onto a gain medium (preferably a diode laser). Surface301 and highly reflective surface 302 are formed respectively by therear and emission facets of the diode laser to result in athree-reflector (i.e., reflective surfaces 301, 302, 304) system.Reflective surface 304 may be deposited on a mirror substrate 309.Again, the curvature of surfaces 301, 302, and 304 should be chosen tosupport a stable cavity mode in a manner well known to those skilled inthe art. A suitable technique for forming such reflective surfacesconsists of depositing a dielectric stack mirror onto a substrate, andtransferring the stack to the emission facet (E. Schmidt et al.,"Evaporative coatings," May 1995, pp. 126-128, Photonics Spectra).

FIG. 4B shows an embodiment in which more than one gain medium isoptically locked to the external cavity at the same time, as long as theadditional gain medium or media exhibit a nonlinearity. In FIG. 4B, asystem similar to that of FIG. 2, an additional gain medium 114A iscontained in a second laser cavity 108A, which is defined betweenreflective surface 101A having a reflectivity R₄ and reflective surface102 via beam splitter 103. R₄ can, but need not, be the same as R₁, aslong as it functions in an analogous manner to result in resonance andlight input into the external resonant cavity 110. Likewise, theadditional gain medium 114A and the additional laser cavity 108A can butneed not have the same bandwidth as the first gain medium 114 and thefirst laser cavity 108. In fact, gain medium 114A and laser cavity 108Acan resonate at a frequency different from that of gain medium 114 andlaser cavity 108. The advantage of optically locking more than one gainmedium to the external cavity is that more power or additionalfrequencies may be trapped in the external cavity. Additional gain mediacould be added in the same manner. Beam splitter 103 can be apolarization beam splitter. In another related example, a diode arraycan replace the diode gain medium 214 in a system similar to that ofFIG. 3.

Limiting the Resonant Frequencies

An example of a preferred embodiment having a frequency-limiting deviceis shown in FIG. 5. In general, the gain medium has an amplificationbandwidth that spans many cavity resonant frequencies. In anoptical-feedback laser system, the gain medium may frequency-lock to anyone of the external cavity resonant frequencies. For example, typicalInGaAlP diode lasers have a gain bandwidth of approximately 10 THzcentered about 670 nm, and the external cavity resonant frequencyspacing is 1.5 GHz when the external cavity length is 10 cm. This meansthat the system may lock to any of more than 6,000 possible frequencies.In some applications, such as particle-counting, this frequency range isacceptable, while for other applications, such as some chemical (e.g.,spectral) analysis, nonlinear frequency conversion, or distancemeasurement, the number of possible locking frequencies must be limited(in some cases to fewer than ten, even as few as one). In theseinstances a frequency-limiting device can be employed to filter out theundesirable frequencies. Examples of such devices are described indetail by King et al. (supra). These devices may include one or acombination of gratings, etalon, lyot filters, or dielectric stackfilters. Some applicable filters are available, for example, fromResearch Electro-optics, Boulder, Colo.

The optical length between the second reflector 202 and the thirdreflector 204 is called the cavity length, which must be of a value thatpermits only a single longitudinal mode to return to the diode laserthrough the frequency limiting device 222. This can be accomplished bysetting the cavity length to be small enough such that the longitudinalcavity mode frequency spacing (given by the speed of light divided bytwice the cavity length, i.e., c/(2L) Hz) is larger than the bandwidthof the frequency limiting device.

King et al. also describe how the rear surface of a diode laser gainmedium may be coated with a distributed Bragg reflector, which alsolimits the allowed frequencies of the system. In the apparatus shown inFIG. 5, the surface 201 can be thus coated. Then the cavity length needsto be small enough such that the longitudinal mode spacing is largerthan the bandpass of the Bragg reflector. As an illustrative example, ifthe bandpass of the frequency limiting device or the Bragg reflector is1 nm (corresponding to a bandpass of 668 GHz at 670 nm), the cavitylength must be approximately 200 μm.

As shown in FIG. 5, a frequency-limiting device 222 is placed betweenmode-matching optics 216 and mirror 207 in a system similar to that ofFIG. 3. In this way, the frequency-limiting device 222 produces the mosteffect using a minimum number of components. Such a system has beenconstructed using a Philips CQL801D diode laser as gain medium 214,having its emission facet coated to have a reflectivity in the range of10⁻⁵ to 10⁻⁴. The mode-matching optics 216 consisted of anantireflection-coated (AR-coated) lens with a numerical aperture (NA) of0.48 and focal length of 4.8 mm, an anamorphic prism pair (3:1), and a25 cm focal length lens. Surfaces 202 and 204 had a 17 cm radius ofcurvature, with R₂ =0.9999 and R₃ =0.99999. The length of the externalcavity 210 was 10 cm.

The frequency-limiting device 222 for this example is depicted in FIG.6. It consisted of a metalized mirror, 232, and a 1800 g/mm diffractiongrating (Zeiss) 236, arranged so that the mirror 232 provided a secondpass of the optical beam along the light path 238 on the diffractiongrating, doubling the effective dispersion. The same components could beused to bounce light a large number of times from the grating, thusdecreasing the total system bandwidth. Alternatively, one bounce on thediffraction grating could also be employed. In this system, with a diodecurrent of 65 mA about 230 W, light was generated in the external cavitywith stable system performance.

Another preferred frequency-limiting device is an ultranarrow-bandtransmission filter based on very low loss dielectric stack mirrorsspaced by a half-wavelength-thick layer (Research Electro-optics,Boulder, Colo.). A filter deposited on a 1 inch (2.54 cm) substrate wasused in a system similar to that of FIG. 5. The filter had atransmission of about 80% and a bandwidth of 0.08 nm. This filter wasoperated in a system consisting of an AR-coated Toshiba 9225 diode laser214. The mode-matching optics 216 consists of an AR-coated lens withNA=0.48 and focal length 4.8 mm; a 3:1 cylindrical Galilean telescope(focal lengths +38.1 mm and -12.7 mm); and a 12.5 cm spherical lens 216,with an ultranarrow-band transmission filter as the frequency-limitingdevice 222. Mirrors 207 and 209 from Research Electro-optics each has aradius of curvature of 10 cm. The external cavity length was 8 cm. Thereflectivity R₃ of surface 204 was about 0.99999. Different values of R₂(the reflectivity of surface 202) were employed. The results aretabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                                  diode current                                                                           power (W) in external cavity,                             R.sub.2   (mA)      about                                                     ______________________________________                                        0.99936   72        60                                                        0.99966   78        70                                                        0.99980   69        100                                                       ______________________________________                                    

The advantage of using an ultranarrow-band transmission filter as thefrequency-limiting device 222 is that all the components may be alignedalong a single straight axis 206. In another embodiment theultranarrow-band transmission filter may be deposited directly ontomirror 207 in place of the antireflection coating 219.

In some applications it is preferable that only one or a few externalcavity modes may lase (i.e. resonate). To this end, one can impose anadditional restriction on the ratio of the laser cavity and externalcavity lengths. When operation is restricted by the bandwidth of thegain medium or the frequency-limiting device 222 to only a few modes,the power stability depends on the effective locking range of the gainmedium. In the case of a diode laser, the locking is in part due to theinteraction between gain and phase (since wavelength is determined bythe external cavity) as well as effective reflectivity of the externalcavity (C. H. Henry, et al., "Locking range and stability of injectionlocked 1.54 μm InGaAsP semiconductor lasers," August 1985, vol. QE-21(8), pp. 1152-1156, IEEE Journal of Quantum Electronics). For both theexternal and laser cavities to be resonant at the same wavelength, theoptical-path length in each cavity has to be an integral number of halfwavelengths. The diode laser may adjust its phase delay to match thiscondition by altering the saturated gain (C. H. Henry et al., supra).

It can be shown mathematically that for stable build-up in the lasercavity (and hence frequency-locking) to occur the electric field appearsin the laser cavity with different phase delays at differentexternal-cavity resonant frequencies. The ratio, r, of the laser-cavityand external-cavity lengths can be expressed as r=n+a/b, where n is aninteger, whereas a and b are real numbers. If a=0, the ratio r isintegral. Then the electric field at all the external-cavity resonantfrequencies occurs with the same phase delay, repeating every 2π. Thediode laser has an initial phase delay that may be different from thatof the electric field at any of the cavity resonant frequencies. In thiscase, to remain locked to the external cavity, the maximum amount inphase (i.e., gain) the diode laser has to adjust is ±π. On the otherhand, if a=1 and b=3 and the diode cavity is restricted to lase overonly three modes (e.g., by the frequency-limiting device 222), then themaximum phase adjustment can be shown to be ±π/3. Without afrequency-limiting device, the diode laser may simply lase at adifferent cavity resonant frequency in order to acquire the additionalphase delay.

When the diode laser is restricted to only a few modes, lockinginstability may occur if the diode cannot adjust the phase delay farenough. The nonlinearity responsible for a gain-dependent phase differsbetween diode lasers. In cases when the nonlinearity is small, a smalladjustment of the phase shift is preferable over a large one to maintainstable locking. This effect is shown in FIG. 7 where a system of FIG. 5was used. The gain medium 214 was a Hitachi 6714G laser and thefrequency-limiting device was an ultranarrow-band transmission filter.The threshold current (a measure of the saturated gain) is largerwhenever the laser cavity length is an integral multiple of the externalcavity length (9 cm). In embodiments where the gain medium exhibits alimited locking range (or limited nonlinearity) a nonintegral ratio ofexternal cavity length to laser cavity length is preferred. Preferably,the ratio of b/a is large, more preferably greater than 3.

To make a compact device with a laser build-up cavity of the presentinvention, the first, second, and third reflective surfaces can be madeby machining (such as micromachining) of a substrate (e.g., silicon,silicon dioxide, and the like) and coating with a suitable dielectric(or another suitable reflecting material) to obtain the selectedreflectivity at the desired positions. In this way, the laser cavity andthe external resonant cavity can be formed at the proper positions.Standard machining techniques, including micromachining andmicrolithographic techniques can be used. For example, Jerman et al. ("Aminiature Fabry-Perot interferometer with a corrugated silicon diaphragmsupport," Sensors and Actuators, 29, 151 (1991)) describe how tomicromachine a two mirror cavity. This technique can be used to make thelaser cavity and the external resonant cavity of a three-mirror systemin accordance with the present invention. Furthermore, it iscontemplated that other optical components, such as mode-matchingdevices, can also be formed by such machining techniques. Forming theoptical elements on a substrate (preferably as a unitary, integral unit)obviates the need for securing means such as adhesive, nuts and bolts,screws, clamps, and the like, as well as reduces alignment and movementproblems.

Applications

The present invention can be used advantageously in many applications.Examples include nonlinear frequency conversion and distancemeasurement. Once a suitable laser is provided (e.g., by the presentinvention), such operations can be done with skill known in the art.Intracavity frequency conversion has been described by several authors:for frequency doubling, by E. S. Polzik and H. J. Kimble, "Frequencydoubling with KNbO₃ in an external cavity," September 15, vol. 16 (18),Optics Letters, W. Lenth and W. P. Risk (supra), W. J. Kozlovsky et al.(Supra), and A. Hemmerich et al. (Supra),; and for nonlinear mixing, byP. G. Wigley et al. (Supra), and P. N. Kean and G. J. Dixon, "Efficientsum-frequency upconversion in a resonantly pumped Nd:YAG laser," January15, vol. 17 (2), Optics Letters.

FIG. 8 shows an illustrative schematic view of a system that can be usedto generate optical frequencies other than the frequency supplied by thegain medium 204. A nonlinear crystal 401 is placed inside the externalcavity 110 in a setup similar to that of FIG. 2. The nonlinear crystalconverts the light from the gain medium 114 to light of otherfrequencies. The reflective surfaces 402 and 404 replace surfaces 102and 104 of FIG. 2. Surfaces 402 and 404, in addition to having the samereflectivity ranges as surface 102 and 104 (taking into account theadditional optical loss associated with the passage of light through thecrystal), may be reflective at any of the frequencies of the light thatis nonlinearly generated. One or more crystals may be necessary tocomplete the nonlinear conversion. If needed, several crystals may beplaced in the external cavity 110. In some cases, a frequency-limitingdevice 222 may be used, such as when the nonlinear frequency conversionoccurs over a narrow frequency range and there is no other mechanism torestrict frequency.

In FIG. 8, the crystal surfaces exposed to the light in the light path,106 is preferably antireflection-coated to minimize the bandwidth of theexternal cavity, thereby improving the frequency-locking of the externalcavity to the laser cavity. An alternative, simpler embodiment is shownin FIG. 9, wherein the reflective surfaces 402 and 404 are directlydeposited onto the surfaces of the crystal 401. FIG. 9 shows that adiode laser 214 is used as the optical source. Mode-matching optics 216,and a frequency-limiting device 222, may also be used for optimaloperation.

Optical distance measurement requires a source that generates a stablenarrow bandwidth beam. A suitable source is an embodiment of the presentinvention (e.g., one shown in FIG. 10). In this embodiment, thereflective surfaces 202 and 204 are deposited onto a solid piece ofoptically transparent support material 501. Support materials having lowthermal expansion coefficients, e.g., zerodur or fused silica can beused to increase the thermal stability. However, to adjust the distance(thereby tuning the resonant frequency of the external cavity) betweenthe reflective surfaces 202 and 204, the solid support 501 can bethermally controlled to expand or contract it. Means for thermallycontrolling are well known in the art. Also, the reflective surfaces 202and 204 can be supported by a support not integral with (i.e., it is notthe substrate on which these surfaces are coated). Such thermalcontrolling tuning of resonant frequency can also be applied in a systemdescribed in FIG. 9.

As previously stated, the light made available with the presentoptically locked external cavity (especially high intensity light) iseffective for use in chemical sensing (analysis). For example, in FIG.11, which shows a laser system 506, a sample 503 containing targetanalytes can be placed in the beam path 106 in the external resonantcavity 110 to cause light interaction (e.g., light absorption, lightscattering, Raman scattering, fluorescence, indirect fluorescence,phosphorescence, and the like). A detector 505 can be positionedadjacent to the sample 503 to sense the light interaction, therebyproviding analytical data on the analytes in the sample 503. The samplecan be placed in the beam path by means of a container 507 that does notsubstantially absorb or reflect light of the desired frequency (orfrequencies). Alternatively, the reflective surfaces 102, 104 can bepart of the structure (e.g., the container) that confines the sample.Another example is depositing the sample on the side of the reflectivesurface 104 exterior to the external resonant cavity 110 such that thelight interaction is caused by evanescent excitation.

Adjusting the Distance Between the Second Reflector and the ThirdReflector

As previously described, to tune the resonant frequency of the externalcavity (for a narrow bandwidth), the distance between the second and thethird reflector can be adjusted by thermal expansion and contraction ofthe structure that supports these two reflectors. FIG. 12 illustrates analternate embodiment which uses a servo mechanism for moving the thirdreflector. Although a servo mechanism is shown only in FIG. 12, it isunderstood that a servo mechanism can be applied to any of the strongfeedback laser systems described hereinbefore wherein the second and thethird reflectors are not unmovably affixed relative to each other.

In FIG. 12, a servo mechanism is incorporated into a laser systemsimilar to that of FIG. 5. This servo mechanism 511 includes apiezoelectric stack 512 operatively connected to the mirror 209 of thethird reflective surface 204 (i.e., connected to the substrate on whichthe reflective surface is deposited). This piezoelectric stack 512 is inturn connected to the appropriate electrical drive (not shown in FIG.12) for driving it to cause motion. In this way, the distance betweenthe second and the third reflective surfaces can be adjusted to tune theresonant frequency of the external cavity 210.

The wavelength of the light in the external cavity can be measured byexamining the emission through surface 204 (and mirror 209) or surface201 with a light analyzer 513 which measures the wavelength (orfrequency). Such wavelength (or frequency) measuring devices are knownin the art and include a grating spectrometer, or alternatively, anetalon. (See also, e.g., Kuntz et al., "Miniature integrated-opticalwavelength analyzer chip," Optics Letters, 20, p. 2300. (1995).) Inaddition, an electronic feedback system (or device) 515 can also be usedto control the piezoelectric stack drive based on feedback from thewavelength measuring device to result in a desired wavelength from thegain medium 214.

One important advantage of a laser system of the present invention isthat the spatial mode quality is very high because the second and thethird reflective surfaces can be made with an appropriate curvature suchthat only one spatial mode can be supported. The narrow-band radiationcan be accessed by analyzing the leakage through surfaces 204 or 201.Alternatively, the light inside the external cavity can be analyzed, forexample, by Doppler-free spectroscopy. (See, M. D. Levenson,Introduction to Nonlinear Laser Spectroscopy, Academic Press, New York,1982. P 164.)

Although the illustrative embodiments of the device of the presentinvention and the method of using the device have been described indetail, it is to be understood that the above-described embodiments canbe modified by one skilled in the art, especially in sizes and shapesand combination of various described features, without departing fromthe spirit and scope of the invention. For example, various featuresdisclosed in this application can be combined.

What is claimed is:
 1. A tunable laser system comprising:(a) anoptically nonlinear laser gain medium; (b) a first reflector having areflectivity (R₁) and a second reflector spaced from the first reflectorto define a laser cavity containing the laser gain medium, the secondreflector having a reflectivity (R₂) larger than the reflectivity (R₁)of the first reflector, such that the laser gain medium amplifies lightin the laser cavity; and (c) a third reflector having a reflectivity(R₃) larger than the reflectivity of the first reflector (R₁), spacedfrom the second reflector to define therewith a resonant cavity externalto the laser cavity, such that light passes from the laser cavity toresonate in the external resonant cavity and light passes from theexternal resonant cavity to optically lock the laser gain medium bymeans of the nonlinearity of the laser gain medium, the distance betweensaid third reflector and said second reflector being adjustable to tunethe resonant frequency of the resonant cavity.
 2. The system of claim 1wherein the laser gain medium is a laser diode which is caused to lockto a resonant frequency of the external resonant cavity by strongoptical feedback to the laser diode from the external resonant cavity.3. The system of claim 1 wherein the laser gain medium is a laser diodewhich is caused to lock to a resonant frequency of the external resonantcavity by optical feedback to the laser diode of more than 10% of thelight transmitted from the laser diode to the external resonant cavity.4. The system of claim 1 wherein the light intensity in the externalresonant cavity is at least one order of magnitude larger than that inthe laser cavity.
 5. The system of claim 1 wherein R₁ is from 0.99 to0.1, R₂ is from 0.9 to 0.999999, and R₃ is from 0.9 to 0.999999.
 6. Thesystem of claim 1 wherein R₁ is from 0.99 to 0.1, R₂ is from 0.995 to0.999999, and R₃ is from 0.995 to 0.999999.
 7. The system of claim 1further comprising a frequency-limiting device between the laser gainmedium and the external resonant cavity to limit the range of frequencyemitted into the external resonant cavity.
 8. The system of claim 1wherein R₃ is larger than R₂ which is larger than R₁.
 9. The system ofclaim 1 further comprising a nonlinear optical element disposed betweenthe second reflector and the third reflector to convert light from thelaser gain medium to a different frequency and wherein the second andthe third reflectors are adapted for light of the different frequency toresonate.
 10. The system of claim 1 wherein the laser gain medium has anantireflection-coated facet spaced from and facing the second reflector.11. The system of claim 1 wherein the laser gain medium has twoantireflection-coated facets spaced from the first and secondreflectors.
 12. The system of claim 1 further comprising mode-matchingoptics to spatially match the light emitted from the laser gain mediumto the external resonant cavity.
 13. The system of claim 1 wherein thesecond reflector is deposited directly on a facet of the gain medium.14. The system of claim 1 wherein a servo mechanism is operativelyconnected to the third reflector for adjusting the distance between thesecond reflector and the third reflector.
 15. The system of claim 1wherein a piezoelectric stack is operatively connected to the thirdreflector for adjusting the distance between the second reflector andthe third reflector.
 16. The system of claim 1 wherein the secondreflector and the third reflector are connected to a common structurewhose thermal expansion can be controlled to adjust the distance betweenthe second reflector and the third reflector.
 17. A tunable laser systemcomprising:(a) an optically nonlinear laser diode; (b) a first reflectorhaving a reflectivity (R₁) and a second reflector spaced from the firstreflector to define a laser cavity containing the laser diode whichamplifies light in the laser cavity, the second reflector having areflectivity (R₂) larger than the reflectivity (R₁) of the firstreflector; (c) a third reflector having a reflectivity (R₃) larger thanthe reflectivity of the first reflector (R₁) and defining with thesecond reflector a resonant cavity external to the laser cavity, suchthat light passes from the laser cavity and resonates in the externalresonant cavity along a resonant intracavity beam path and light passesfrom the external resonant cavity into the laser cavity to opticallylock the laser diode by means of the nonlinearity of the laser diode,the distance between said third reflector and said second reflectorbeing adjustable to tone the resonant frequency of the resonant cavity;(d) a means associated with the external resonant cavity for exposing ananalytical sample to light energy from the external resonant cavity toresult in light interaction characteristic of an analyte in theanalytical sample; and (e) a detector positioned adjacent to the meansfor exposing to detect the light interaction; such that the laser systemis adapted for detecting the presence of the analyte in the analyticalsample.
 18. A method for locking an optically nonlinear laser gainmedium, comprising:(a) emitting a light beam from the laser gain mediumdisposed in a laser cavity defined by a first reflector and a secondreflector spaced from the first reflector such that the laser gainmedium amplifies light in the laser cavity; and (b) transmitting lightfrom the laser cavity to a resonant cavity defined by the secondreflector and a third reflector, the resonant cavity being external tothe laser cavity, such that light emitted from the laser gain mediumresonates in the external resonant cavity, part of the light in theexternal resonant cavity being transmitted back to the laser cavity tooptically lock the laser gain medium by means of the nonlinearity of thelaser gain medium to a resonant frequency of the external resonantcavity; and (c) adjusting the distance between the second reflector andthe third reflector to tune the resonant frequency in the externalresonant cavity.
 19. The method of claim 18, wherein the first reflectorhas a reflectivity (R₁), the second reflector has a reflectivity (R₂)larger than the reflectivity (R₁) of the first reflector, and the thirdreflector has a reflectivity (R₃) larger than the reflectivity of thefirst reflector (R₁) such that light resonates in the external resonantcavity to build up power therein to one or more orders of magnitudelarger than the light in the laser cavity.
 20. A method of making anoptically locked laser system, comprising:(a) positioning a firstreflector having a reflectivity (R₁) a distance away from a secondreflector to form a laser cavity, the second reflector having areflectivity (R₂) larger than the reflectivity (R₁) of the firstreflector; (b) positioning an optically nonlinear laser gain medium inthe laser cavity such that the laser gain medium can amplify lighttherein; and (c) forming an external resonant cavity by positioning athird reflector spaced from the second reflector and external to thelaser cavity, the third reflector having a reflectivity (R₃) larger thanthe reflectivity of the first reflector (R₁), such that light emittedfrom the laser gain medium can enter and resonate in the externalresonant cavity along a resonant intracavity beam path; wherein part ofthe light resonating in the external resonant cavity reenters the lasercavity from the external resonant cavity to lock the laser gain mediumto a resonant frequency of the external resonant cavity by opticalfeedback by means of the nonlinearity of the laser gain medium, and suchthat the distance between the second reflector and the third reflectoris adjustable to tune the resonant frequency of the external resonantcavity.